The present invention relates to improved coatings and a method of applying these coatings to adsorption heat exchangers and temperature controlled adsorbers. In particular, the present invention provides for a method of providing thin adsorbent coatings that allow for much higher operating efficiency, greater heat transfer capacity and more effective functionality than prior art adsorbent packed beds.
There are a variety of solid adsorbents which have been useful in adsorption and catalysis including commonly known materials such as activated carbons, activated clays, silica gel, activated alumina, and crystalline molecular sieves. Of these adsorbents, crystalline molecular sieves such as silicoaluminophosphates, aluminophosphates and aluminosilicate zeolites have been particularly useful because of their uniform pore size.
In many instances it is desirable to have the solid adsorbent deposited on a substrate as a coating instead of being contained in particulate form as pellets, beads, or other particles. There are several reasons why solid adsorbent coatings have been used including, for example, to improve the catalytic or adsorption properties of the solid adsorbent by improving the surface area to weight ratio, to reduce pressure drop, to enhance mass transfer rates, to reduce the amount of solid adsorbent required, to protect the underlying substrate material from a harmful environment, to achieve a particular strength or form, and, to perform the particular adsorptive or catalytic function over the entire coated surface of the substrate.
Not surprisingly, there has been a diversity of end users for solid adsorbent coatings. Moreover, the methods for applying the coatings have been varied and somewhat dependent on the particular end use. The use of adsorption heat exchangers was disclosed in US 2008/0023181 A1. In that invention, a metal substrate is coated with an adsorptive material coating in accordance with U.S. Pat. No. 5,120,694 which taught the use of ceramic binders together with zeolite and other adsorbents as the coating material that are applied in a water based slurry. These prior art heat exchangers contain high quality adsorbent layers but these materials provide an intrinsic limit where it is not possible to have a density of fins more than about 197 to 276 fins per linear meter (5 to 7 fins per linear inch). Increased density of fins is desirable in providing for increased mass transfer.
U.S. Pat. No. 6,576,199 discloses the manufacture of an aircraft environmental control system in which a metal catalyst is placed upon a catalytic converter with a wash coat that contains an organic siloxane resin. This is a very thin layer that is provided to enhance catalytic activity as compared to the present invention where adsorption and effective heat transfer are the result of providing the layer of adsorbent material on the substrate.
Whereas the ceramic structure of the prior art does promote good mass transfer the art taught here imparts superior mass transfer due to the framework structure that is formed with the inorganic component (SiO2) that is left behind after the curing and calcination of the organosiloxane resin. We further anticipate the need to coat and the ability to coat substantially larger adsorption heat exchangers and teach methods whereby the coating can be applied to large heat exchangers which could not be dip coated via the means taught in prior art.
Adsorption heat exchangers or temperature controlled adsorbers can achieve much higher operating efficiency, greater heat transfer capacity and more effective functionality as both an adsorber and a heat exchanger if the adsorbent is applied as a coating on the heat exchanger surface rather than by including the adsorbent as a “packed bed” of beads, pellets or granules of adsorbent material.